LEDs unlike conventional light sources such as incandescent bulbs cannot effectively cool themselves. As such additional appended heatsinks or cooling means are required to prevent overheating. This increases the cost of not only the light sources due to shipping costs and materials costs but also the fixtures that use those light sources. It also results in heavy light source fixtures. In general, the need exists for articles and means which allow LEDs to be used without the need for additional heatsinking means.
It is desirable to minimize the temperature difference between the junction or active region of the semiconductor device and the ambient atmosphere to effectively cool small semiconductor devices. It is also desirable to minimize the surface area needed to dissipate the heat generated by the semiconductor devices to the ambient atmosphere. While high thermal conductivity materials can be used to spread the heat out over a very large area, these high thermal conductivity materials come with the addition of significant weight and cost. In conventional LED devices several layers of interconnect exist between the LED die and the final light source. This approach is used because the lighting fixture manufacturers have historically not been required or had the capability to wirebond, flip chip attach or even solder components into their fixtures. Also the need to regularly replace light sources such as incandescent bulbs has led to a wide range of quick change interconnects like sockets and pin based connector. Lightweight self cooling solid state light sources would offer significant benefits to fixture manufacturers. Incandescent bulbs for instance are very lightweight generating over 1000 lumens while weighing only 50 grams and as such can be easily held in place using even simple pins and sockets. For the typical LED sources, this is not the case. The added weight of the heatsink and the need for a low resistance thermal connection between the LED package and the heatsink necessitates the use of complex multiple level interconnects. The need exists for LED light sources which are lightweight and easily incorporated into a wide range of lighting fixtures without the need for additional heatsinking or cooling means.
Historically, light sources have cooled themselves as stated earlier. In the case of incandescent and fluorescent tubes, the glass envelope surrounding the sources, and the filament or arc itself transfers the excess heat generated via convection and radiation. An incandescent bulb glass envelope can exceed 150° C. and a halogen's quartz envelope may exceed several hundred degrees Celsius. Radiative power scales as the fourth power of the temperature. A naturally convectively cooled surface with a surface temperature of 50° C. in a 25° C. ambient will transfer only about 5% of its energy to the surrounding ambient radiatively. A naturally convectively cooled surface with a surface temperature of 100° C. can transfer 20% of its energy to the surrounding ambient radiatively. The typical LED junction temperature for high powered devices can be over 120° C. and still maintain excellent life and efficiency. For surfaces with temperatures less than 120° C. the majority of the radiated energy is in the infrared with a wavelength greater than 8 microns.
Heat generated within the LEDs and phosphor material in typical prior art solid state light sources is transferred via conduction means to a much larger heatsink usually made out of aluminum or copper. The temperature difference between the LED junction and heatsink can be 40 to 50° C. The temperature difference between ambient and heatsink temperature is typically very small given the previously stated constraints on the junction temperatures of LEDs. This small temperature difference not only eliminates most of the radiative cooling but also requires that the heatsink be fairly large and heavy to provide enough surface area to effectively cool the LEDs. This added weight of the heatsink increases costs for shipping, installation and poses a safety risk for overhead applications. For example lighting in a typical industrial or office building will use troffers. These troffers which are typically 2 foot by 4 foot house fluorescent tubes and weigh as much as 30 pounds including the electronic ballasts. The four foot fluorescent tubes by themselves weigh 200 grams each. These troffers have to be separately rigged and supported independent of the suspended ceiling. They pose a safety hazard in the event of a severe earthquake. They also typically pose a fire hazard as the diffusing elements which interface to the occupant side of the room are made out of flammable materials (e.g. plastic). In newer installations light emitting diode (LED) based solid state troffers are being use to replace fluorescent troffers. These solid state troffers however still require large and heavy appended heatsinks to dissipate the excess heat from the LEDs. They also use large plastic diffusers to spread the light out over a larger surface.
Surprisingly, much like conventional incandescent, halogen and fluorescent light sources, conventional solid-state light source are not typically flame resistant or even conform to Class 1 or Class A building code requirements. There are two types of fire hazards indirect (where lamp/fixture is exposed to flames) and direct (where the lamp/fixture creates the flames). Conventional solid-state lamps and fixtures can pose both indirect and direct fire threats because they use large quantities of organic materials that can burn.
Even though the LED die are made using inorganic material such as nitrides or AlinGaP which are not flammable, these LED die are typically packaged using organic materials or mounted in fixtures which contain mostly organic materials. Organic LEDs or OLEDs not only are mostly organic but also contain toxic materials like heavy metals like ruthenium which can be released if burned. Smoke generated from the burning of these materials is not only toxic but one of the leading causes of death in fires due to smoke inhalation. Incandescent and fluorescent lighting fixtures typically are composed of sheet metal parts and use glass or flame retardant plastics designed specifically to meet building code requirements. It is therefore advantageous that solid state light sources be constructed of non-flammable and non-toxic materials especially in commercial applications like suspended ceilings. This is for the benefit of both for occupants and firefighters. Organic materials containing heavy metals and nanoparticles such as quantum dots are especially problematic.
As an example, solid-state panel lights typically consist of acrylic or polycarbonate waveguides which are edge lit using linear arrays of LEDs. A couple of pounds of acrylic can be in each fixture. Integrating these fixtures into a ceiling can actually lead to increased fire hazard.
Other troffer designs rely on large thin organic films to act as diffusers and reflectors as seen in recent LED troffer designs. During a fire these organic materials pose a significant risk to firefighters and occupants due to smoke and increased flame spread rates. In many cases, the flame retardant additives typically used to make polymers more flame retardant that were developed for fluorescent and incandescent applications negatively impacts the optical properties of waveguides and light transmitting devices. Class 1 or Class A standards cannot be met by these organic materials. As such a separate standard for optical transmitting materials UL94 is used in commercial installations. The use of large amounts of these organic materials in conventional solid-state light sources greatly increases the risks to firefighters and occupants due to their high smoke rate and tendency to flame spread when exposed to the conditions encountered in a burning structure. A typical commercial installation with a suspended ceiling contains 10% of the surface area as lighting fixtures. The ceiling tiles are specifically designed to act as a fire barrier between the occupants and the plenum above the suspended ceiling. The lighting fixtures compromise the effectiveness of this fire barrier by providing a pathway for flames to bypass the ceiling tiles. For this reason even incandescent and fluorescent fixtures are typically required to have additional fire resistant covers on the plenum side of the ceiling.
These fire enclosures increases costs and eliminates the ability to effectively cool the light fixture from the plenum side of the ceiling. Given that most solid state troffers depend on backside cooling these fire enclosures lead to higher operating temperatures on the LED die and actually increase the direct fire hazard for solid state light sources. The large amount of organics in the solid state light fixtures can directly contribute to the flame spread once exposed to flames either indirectly or directly. The need therefore exists for solid state lighting solutions which are Class 1 rated which can reduce the risks to occupants and firefighters during fires and minimize the direct fire hazard associated with something failing with the solid state light bulbs.
The recent recalls of solid-state light bulbs further illustrate the risks based on the solid-state light sources themselves being a direct fire hazard. In the recalls, the drive electronics over-heated, which then ignited the other organic materials in the light source. The need exists for solid state light sources which will not burn or ignite when exposed to high heat and even direct flames. Existing incandescent and fluorescent lighting fixtures have over the last several decades found that the ideal solution is to construct the majority of the fixture using inorganic materials and to maximize the lumens per gram of the source. A typical incandescent source emits greater than 30 lumens per gram and the source is self cooling based on both convective cooling and radiative cooling. A conventional solid-state light bulb emits less than 5 lumens per gram and requires heatsinking means to transfer the heat generated by the LEDs and drive electronics to the surrounding ambient. The heatsink surfaces must be exposed to the ambient.
In many cases such as recessed can lights the heatsink surfaces are enclosed which dramatically reduces the heat that can be transferred to the ambient. The high lumen per gram in the incandescent and fluorescent bulbs also translates directly into less material to burn both indirectly and directly. Also, in solid-state light bulbs the drive electronics and light source have the same cooling path and therefore heat generated in the drive electronics is added to the heat generated by the LEDs. The added heat from the LEDs elevates the temperature of the drive electronics and vice versa. In the recalls this has led to catastrophic results igniting the organic materials used in the solid state light sources. The coupling of the heat from the drive electronics and the LEDs combined with the large quantity of organic materials used creates a direct fire hazard when components like polymer capacitors and organic coated wiring overheat and burn. Based on years of effort the incandescent and fluorescent sources have moved away from organic based materials for exactly the reasons illustrated above. The solid state lighting industry needs to develop high lumen per gram solid state light sources which not only improve efficiency but also do not represent a fire hazard either indirectly or directly.
Commercial light applications are also subject to seismic, acoustic, and aesthetic requirements. Seismic standards require that suspended ceilings withstand earthquake conditions and more recently these same requirements are being used to address terrorist attacks. In general, lighting fixtures must be separately suspended from the overhead deck in suspended ceiling applications because of their weight and size. The need exists for solid state lighting solutions which can be integrated and certified with suspended ceilings. Regarding acoustics the suspended ceiling dampens noise levels by forming a sound barrier in a manner similar to the fire barrier previously discussed. The lighting fixtures again compromise the barrier created by the ceiling tiles because they cannot be directly integrated into the ceiling tiles or grid work. The need exists for solid state lighting sources which do not degrade the acoustic performance of the ceilings. Lastly, lighting is aesthetic as well as functional. Market research indicates that troffers while functional are not desirable from an aesthetic standpoint. The need therefore exists for solid state lighting sources which provide a wider range of aesthetically pleasing designs.
Suspended ceiling represent a large percentage of the commercial, office and retail space. In this particular application 2 ft×2 ft and 2 ft×4 ft grids are suspended from the ceiling and acoustic/decorative tiles are suspended by the t shaped grid pieces. Lighting has typically been 2×2 or 2×4 troffers which similarly are suspended on the t shaped grid pieces. The troffers are wired to the AC bus lines above the suspended ceiling. Each troffer consists of a sheet metal housing, driver, light sources, and reflective and diffusive elements. In the case of solid state troffers additional heatsinking means or cooling means may also be incorporated into each troffer. To comply with building codes most fixtures require additional fire containment housings which isolate the lighting fixture from the plenum space above the suspended ceiling. In general a standard troffer requires a minimum volume of 1 cubic foot for a 2×2 and 2 cubic feet for a 2×4. The typical lumen output is 2000 lumens for a 2×2 troffer and 4000 lumens for a 2×4. In many instances the location of the light fixtures are put on a regular spacing even though uniform lighting throughout the area may not be required or desirable. This is driven by the difficulty and costs associated with relocating the troffers once installed. This leads to excess lighting with its associated energy losses. The need exists for lightweight diffuse and directional lighting fixtures for suspended ceilings that can be relocated easily and upgraded or changed as technology advances.
Recently Armstrong World Industries has introduced its 24 VDC DC FlexZone™ grid system. The T-shaped grid pieces provide 24 VDC connections on both the top and bottom of the grid pieces. The availability of 24 VDC eliminates the need for a separate drivers and ballasts for solid state lighting. The elimination or simplification of the driver allows for very lightweight and low volume light fixtures especially for the cases where self cooling solid state light sources are employed. Lightweight and low volume, translate directly into reduced raw material usage, fixture cost, warehousing costs, and shipping costs. By eliminating fixed metal housings and replacing them with modular and interchangeable optical and lighting elements that directly attach to an electrical grid system like Armstrong's DC FlexZone system costs can be reduced not only for the fixture itself but also for the cost associated with changing the lighting. Close to 2 billion square feet of commercial and retail suspended ceiling space is remodeled or created each year. The need exists for more flexibility in how this space can be reconfigured. Present fixtures require addition support to the deck of the building due to weight and size constraints per seismic building codes. The need exists for field installable and user replaceable lighting fixtures that can be seismically certified with the grid so that the end user can adjust and reposition fixtures as the need arises. Under the present requirements, any changes to the lighting require that the ceiling panels be removed and at a minimum additional support wires must be installed to the building deck before the fixture can be repositioned. This may also require a reinspection of the ceiling in addition to the added cost for the change. The need exists for lightweight, robust lighting that can be easily adjusted by the end user without the need for recertification and outside labor.
In evaluating the weight of light modules it is useful to utilize the concept of lumens per gram. The lumens per gram of light fixtures can have a major impact on manufacturing costs, shipping costs, and storage costs due to reduce materials costs and handling costs. It could also allow for fixtures which can be directly attached to the grid of a suspended ceiling and still meet seismic standards without requiring additional support structures which are commonly needed for existing troffer type light sources
The need also exists for aesthetically pleasing high lumen per gram light fixtures. For many applications, the lighting should be present but not draw attention to itself. This is not the case with troffers which immediately draw attention away from the other parts of the ceiling. Therefore, there is a need for lightweight and compact lighting fixtures which address the above needs in suspended ceiling applications. Again the thickness of the lighting module has a direct impact on the aesthetics of the installation. Existing linear solid state sources require large light mixing chambers to spread the light emitted by the LEDs. This dramatically increases the depth of these light sources. In order for light panel modules to have a an emitting surface close to the plane of the ceiling and not to protrude into the room or office space below, the major portion of the light source module must be recessed into the suspension ceiling. The need exists for low profile, or thin lighting panels with thicknesses under 10 mm, which are attachable to the electrified grids. Ideally these lighting panels would be field replaceable from the office space side of the installation by end users (and not require custom installers) and present an aesthetically pleasing and monolithic and uniform appearance. Essentially the ideal suspension ceiling lighting system would “disappear” into the ceiling from an aesthetic standpoint.
Finally the need exists for solid state lighting source which can meet or exceed Class 1 or Class A standards, meet seismic requirements, meet acoustic standards, be field adjustable, and be easily integrated in an aesthetically pleasing manner into commercial lighting applications. Intelligent lighting allows for integration of lighting and sensors into the lighting system.
Lighting is required for all occupied areas and active control of lighting via light harvesting and occupancy actually can lead to larger energy savings than the conversion from incandescent to solid state lighting. Presently lighting is a separate market and supply chain from security, point of sale, and HVAC. As intelligent systems permeate into retail, offices, manufacturing, and homes existing lighting suppliers may well be replaced by network suppliers.
The need exists for lighting solutions which enable the integration of sensors and networking in a wide range of installations.
As a large portion of the lighting market is based on upgrades, the need exists for retrofit systems that can be attached, mounted or otherwise adhered to a wide range of surfaces. Incandescent and halogen lighting require thermal isolation from combustible surfaces, fluorescent requires high voltage operation and is susceptible to overheating and cold temperature issues. Existing solid state solutions either have limited lumen output or require heatsinking or other cooling means such as fans to operate. Alternately panel based solid state lighting uses waveguide or led array approaches to create distributed light sources. Waveguides are inherently flammable and represent a significant flame spreading issue along with high cost and weight. LED arrays transfer the heat generated into the mounting surface, which can present a significant fire hazard. The need exist for retrofittable solid state light sources which overcome the deficiencies listed above.
This invention discloses a self cooling solid state light source, which overcomes these issues.